The main objective of our work is to investigate the leukomogenic activity of the bovine C-type virus commonly known as BLV. Seroepidemiological surveys have shown that BLV infection is closely associated with leukemia and leukemia risk in cattle. Results of transmission experiments in sheep have provided some evidence for the leukemogenic activity of BLV. However the lack of proper experimental controls in these later studies have precluded an accurate conclusion. Consequently, we began a transmission study in newborn sheep using as inoculum well-characterized bovine lymphoid cells infected with BLV. More recently, the successful propagation of the virus in bat cells made it possible to add to our experiment another group of sheep which was inoculated with large quantities of BLV. As control we included a group inoculated with BLV-free bovine lymphocytes and a group of non-inoculated sheep. The recipients are being monitored at regular intervals for the presence of the virus. The recipients are being monitored at regular intervals for the presence of the virus, antibodies to the internal and envelop BLV antigen, lymphocytosis and leukemia. Chromosome studies of leukemic cells from sheep receiving BLV-infected bovine lymphocytes are carried out in order to accurately determine whether the leukemias are of host origin. The availability of an abundant source of BLV and of a recently developed assay that permits us to establish its infectivity have provided a rational basis for studies on the ability of BLV to induce leukemia in its natural host. In this study we are using as recipients newborn calves obtained from herds carefully characterized with regard to leukemia risk, natural BLV infection, and persistent lymphocytosis.